Fuel cells have been proposed as a power source for electric vehicles and other applications. One such fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell which is also known as a Solid Polymer Electrolyte (SPE) fuel cell. PEM/SPE fuel cells are well known in the art, and include a so-called "membrane-electrode-assembly" (MEA) comprising a thin, solid polymer membrane-electrolyte having a pair of electrodes (i.e., an anode and a cathode) on opposite faces of the membrane-electrolyte.
In the fuel cell, the MEA is sandwiched between a pair of electrically conductive elements (i.e., electrode plates) which serve as current collectors for the electrodes, and contain a so-called "flow-field" which is an array of lands and grooves formed in the surface of the plate contacting the MEA. The lands conduct current from the electrodes, while the grooves between the lands serve to distribute the fuel cell's gaseous reactants (e.g., H.sub.2 & O.sub.2 /air) evenly over the faces of the electrodes. A thin sheet of porous (i.e., about 80%-90% voids) paper, cloth or felt made from graphite or carbon is positioned between each of the electrode plates and the electrode faces of the MEA to support the MEA where it confronts grooves in the flow field, and to conduct current therefrom to the adjacent lands.
The membrane-electrolytes for SPE/PEM fuel cells are well known in the art. Typical such membranes are described in U.S. Pat. Nos. 4,272,353 and 3,134,697, and in the Journal of Power Sources, Volume 29 (1990), pages 367-387, inter alia. SPE/PEM membranes are proton-conductive polymers which are essentially ion exchange resins that include ionic groups in their polymeric structure, one ionic component of which is fixed or retained by the polymeric matrix and at least one other ionic component is a mobile, replaceable ion electrostatically associated with the fixed component. The ability of the mobile ion to be replaced under appropriate conditions with other ions imparts ion exchange and proton-conduction characteristics to these materials. One broad class of cation exchange, proton-conductive polymers is the so-called sulfonic acid cation exchange resin. In the sulfonic acid membranes, the cation ion exchange groups are hydrated sulfonic acid radicals which are attached to the polymer backbone by sulfonation. The preferred such resin is perfluorinated sulfonic acid polymer electrolyte in which the entire membrane structure has ion exchange characteristics. Such proton conductive membranes may be characterized by monomers of the structures: ##STR1## One commercial sulfonated perfluorocarbon, proton conductive membrane suitable for PEM/SPE fuel cells is sold by E. I. DuPont de Nemours & Co. under the trade designation NAFION.RTM.. Other proton conductive membranes are sold by (1) the Gore Company under the tradename Gore Select.TM., (2) the Asahi Glass Co. under the tradename Selemion.TM. and Aciplex-F.TM., (3) the Tokuyama Co. under the tradename Neosepta.TM., (4) the PALL RAI Corp. under the tradename Raipore.TM., (5) the Sybron Chemicals under the tradename Ionac.TM., (6) the Hoechst Celanese Corp., and (7) the Ballard Co. under the trade designation BAM3G.TM., as well as the Dais, Ionics, and Solvay S.A. After electrodes have been applied to the faces of the membrane-electrolytes they are often assembled into the fuel cell in a dry state, and allowed to swell therein when the cell is operated with hydrated reactants (i.e. H.sub.2 and air).
The anode and cathode electrodes on the opposing faces of the PEM/SPE membrane typically comprise either finely divided catalyst particles (e.g., Pt or its alloys) or finely divided carbon particles having the catalyst on the surfaces thereof. The catalyst particles or catalyst-bearing carbon particles are dispersed throughout a polymeric binder or matrix which typically comprises either a proton conductive polymer and/or a fluoropolymer. When a proton-conductive material is used, it will typically comprise the same proton-conductive polymer as makes up the membrane electrolyte (e.g., NAFION.RTM.). The fluoropolymer typically comprises polytetrafluoroethylene (PTFE), though others such as FEP (Fluorinated Ethylene Propylene), PFA (Perfluoroalkoxy), and PVDF (Polyvinylidene Fluoride) are also used. These polymers create a robust structure for catalyst retention, adhere well to the membrane-electrolyte, aid in water management within the cell, and enhance ion exchange capacities of the electrodes. One such membrane-electrode-assembly and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993 and assigned to the assignee of the present invention.
MEA's have heretofore been made by a number of techniques including applying a slurry comprising the carbon-supported catalyst, the polymer matrix/binder and a suitable liquid vehicle (e.g., H.sub.2 O, methanol, isopropanol, etc.) either (1) directly onto the membrane, or (2) onto a separate carrier or release film from which, after drying, it is subsequently transferred onto the membrane-electrolyte using heat and pressure in a decalcomania process. The latter (i.e., decalcomania) process is quite expensive in that it is slow and requires complicated fixturing to align the decal with the membrane as well as a heated press to effect the transfer. These characteristics of the decalcomania process limit the ability to make MEAs in mass production. Applying the slurry directly to the membrane, on the other hand, has potential for a low cost, mass production process and has been disclosed in such patents as Grot et al U.S. Pat. No. 5,330,860; Swathirajan U.S. Pat. No. 5,316,871; and Wilson U.S. Pat. No. 5,211,984. However, it has been found that the vehicle used to slurry the electrode materials causes swelling of the membrane to which it is applied. In this regard, all ion exchange membranes-electrolytes useful for MEA's swell in one or more directions at different rates in different vehicles used for the electrode slurry. These membrane-electrolytes can readily absorb so much of the vehicle (e.g., hydrocarbon solvents), that they can swell up to 25% or more in any given dimension. Even membranes, such as Gore Select.TM., which are designed such that the x-y expansion is held to about 1-2%, can still swell 25% or more in thickness. When the membrane absorbs vehicle and swells, it sags, slumps or droops which results in loss of dimensional control of the membrane and handling difficulties during processing, as well as loss of electrode coherence to the membrane-electrolyte. The present invention permits the making of lengths of MEA strip in a continuous, low cost, manufacturing process which (1) does not require any pressure to bond the electrode layers to the intermediate membrane-electrolyte layer, and (2) only enough heat to remove the vehicle and cure the polymer binder/matrix for the catalyst.